Fast neutron dosimeter



Oct. 28, 1952 G. s.. HuRsr FAST NEuTRoN nosIMETER 2 SI'lEETSf--SI-IEET l Filed Feb. 7, 1951 n INVENTOR. Geo/ge 5T Hur-.s

ATTONEV 06t- 28, 1952 G, s, HURST FAST NEUTRON DOSIMETER 2 SHEETS-SHEET 2 Filed Feb. 7. 1951 Meier Neuzran Ene/gy /h Meu INVENTOR. George 6T Huns BY V ATTORNEY Patented ct. 28, 1952 George bi?. y sign."

Sellerstioak; Ridge:Tennwassisnoxums 'Y d-Statesoof Amer "j A haUeiesSteieS M0191@ @met ApplicationFebnuary, 1951; Serittljilloz..2.5!9;'l3l9i..l

(Cl. 25d-83.6)

Ther. present.x invention; re1ates--- to..y radiation murved ngf tihQl:

y. apar.-

measurement-,Land more esnesiallyito method;- and. apparati.; for-e: deter amountlofi. energy; tha

that cell of fast neutrons, oyer.a,.;broad;spectrum of.. energies; e

incident. .neutrons.,fdepends. in; part -iiiponth al.

amount toffenergyI .absorbed by i the icell. andsthe.: distribution '.of the .ionization @produced merel-1.1i..

Accordingly; the amount of-ienergylabsorbed per.;` gram of` animal tissue C.isiaicn as; a measure of radiation dosagel The unit o'f dosage measuremenig*v calledv th'e repi lentgen`. 1uivialent--. physical) is dei'lned as that'amount oradiation 'vvhichf is. absorbed in Atissue -j tojtl'i'el extent M`of* 3 95 ergs pligiail df t'SSilel" d Alpha and beta particles nmay ionize '.atomsjofr" tissue directly, while gamma rays produce vseclondary electronsvwhich ionize the tissue. Fast neutrons collide; elasticallywith hydrogemA nitro..

gen, carbon; andioxy'genfatoms; the maximum amount. offenergybeing transferred to the lighter hydrOgenfnuclei-Lor recoil protons. These particles lose theirge'ner'gyv in the tissue by excitation and ionization. Considering the effect .only-ol--f therst collisionl of arneut-ron, it has been determined that'the dserec'eived per `neutron per om.2 of tissue varies with the neutron energy,

as the quantity: EEiqgfiQ;ifiyglgienje In-the past; vfast neutron dosehas-beeri-inferred fronkth'e -cuirent produced inaaaspecialv ior'r chamrr beifhaving-a11- its wailis-fiined'witnfa seiidfhydrogenous material and-'its 'volume' ii-lleidfwithea gas lcontainingghydrogen in thesame 'proprt' nl asfthe. Wall Cristina' However.' the 'rr f .rais immun pany a fast neutron beam v Vvca not ,beeliminatedv in such .inteeratngldevices sojah va'd'diti rial lion chamber. sensitive @only to gammaf,.radiation,. is; customarily provided and,A the] gamma .i-in'duc'ed current thereinis subtracted from the'neutron'-u plus. gamma .induce'dr current in the'first cham,- ber. In operation, such devices are euml)llfsomejiv have.l twice. as .rnany-J components .to l.builgifvand maintain-asssinslachemberSf-ang milite-.impara Y to. expose each chamber tothe sarnegamma'ux. With a knowledge'zafithe shortcomings of the object-bf .myrinyentio l .1p me. .hedor iand apparatus .for-.fmQaSur-inggthe:d0

age, or energy absorbed in tissue from fast tronsis..

Another object of my invention is to proyiele-.`

atnovel method of :andgiapparatus for measuring the. ias-t neutron. dosage. t0. tissue. in4 thapresenc. ofgamma radiation.'

Yet another object of my invention is'to provide a novel recoil .zproportionalcounter system which.will.determinetne.dosagem.humantissue fromirraz'diatioii.Withgagfast tr n'beain.""""" An'addiunai'object et j vide a proportional counter'comprisir'ig l'a hydrogenous solid Wall of k"a*predieterr'nined composition and; tl'liigclgnersds andga. hydrogenous iillirig gasa. wherein the composition and thicknessijhe.;

Wall, the length of the counter, and the compo# sition andfpressureof .the-gasrareelated in an especial, predeterminedfmanner such 4that the counting.: IeSDQflSeeO & .00.l SimQaSuPe-'Of theifastfneutrondos fztissue l I Still another object of my invention is.to.;oro-v vide a method of and apparatus for measuring fast neutron dosage'in thep'i'esence of gamma radiation with` buta.single acounten': ;and.;without the necessity for complicated electronic circuitry, byv a Countingrate method.

Ofther `ob`jects1and advantagesiof my invention will be apparent from the following detailed description of a preferred embodiment thereof, whenl readvvi-th:referenceJ ta the .appendedgdrawfn ings, i-n-which lFigure .f1 i's acurve :showings th'eirelative; res scribedhereiznaiten 3 Referring now to Figure 1, curve A shows the calculated response of a plane slab of hydrogenous material to a collimated beam of fast neutrons over the energy spectrum up to m. e. v. The number of proton recoils caused by the neutron beam in an incremental-thickness At is deter-V mined by the area of the slab A1, the number Q of hydrogen atoms per unit Volume of the material, and the scattering cross-section a(E) ofV hydrogen. Of the protons originating at a depth t below the surface of the slab, only a fractional part F(t, E, B) reach the surface with a minimum residua1 energy B, which is assumed 'to be the minimum energy that will be detected in the counter. Therefore, the number ANi, or protons originating in an incremental thickness At which will be counted is:

AN1==A1Qa(E)F,(t, E, B) At (1) If the neutron-proton scattering is assumed to be isotropic, then it may be shown that where @(t, E, B) is the angle withthe normal to the surface which the proton path must make if the particle is to reach the surface with an energy B. v

Assuming now that the range R of protons inl the solid is: Y

R'==K1Epn (3) where Ep is the energy of the proton, and K1 and n are constants, then t=K1(E9-B)cos 0 (4).

where E0, the energy of a proton traveling at the angle 0 with the normal to the surface, is related to the neutron energy E by Ea=E cos2 0 y (5) The number of protons detected per unit time is then Carrying out the integration above, Where t' is either the actual slab thickness or the function defined'by Y Y I y t=K1tEnBni 7) whichever is smaller. we obtain the expression:

energy of a proton originating at the back of the slab and emerging at the front with an energy B.

" ton recoils.

However, in the face of this seeming impasse, I have found that I can, by certain novel arrangements and improvements, provide an accurate fast neutron dosimeter. First, I employ for my counting gas in the detector a hydrogen-bearing gas at a selected. pressure so that the chamber contains onlyv a certain predetermined number of hydrogen atoms per unit volume; second, I provide an additional thick vhydrogenous proton radiator or slab, the insidesurface (away from the source of neutrons) Vof which is covered by a thin metal absorber foil; and third, I measure dosage as a function of the' rate of occurrence of pulses within the detector, rather than the integrated ion current, as has been heretofore done in ionv chambers and the li-ke. l.

If the number of protons scattered vby unit neutron flux incident upon a volume containing C hydrogen atoms is represented by CME), the

fractional part that recoils with an energy not less than B will bell-B/El. The response Na of a counter filled with the gas is then:

N3-of a thick slab of parafn, to thev back ofl which is placed an aluminum foil, to fast neutrons is:

E n 21H-3 E Y Wild-) v 2n+1 Eg) (2n-t1) (12) 21H-3 E provided E E3, E3 B, and Ea=(t3/7c;i)1/1l (13) where t3 is the thickness of the fon,

A3 is the area of the radiator, 1

K3 is a constant relating the range inthe foil of protons to their energy (Ep)n. 1

Equation 12 is plotted as curve C, Figure 1.

- Curve D, Figure 4, shows how'the dose to tissue per unit neutron flux varies with fast neutron energy. Curve E, plotted also in Figure 4, shows the response in counting rate of my dosimeter, and how, by combining the counts available* from a plane hydrogenous slab, a predetermined num? ber of gaseous atoms, and a metal-backed thick hydrogenous slab in one proportional counter,

I have provided an instrument, the response of which to fast neutrons is in very close agreement' with the dose to tissue over the entire neutronv (11) Curve B of Figure 1 illustrates that response, asl opposite directions, have not been K considered.

apte-,osa

'ventlon may comprise a 'brass cylinder.. I, closedat` opposite ends by brass plates 2, 3. Centerwire 120 apart within the outer shelly I, engagingl correspondingly threaded holes in end plates 2, 3-. Nuts I4 may be tightened to insure agas-tight t between the end plates and the outer shell. Plates 3, 9 are carried upon the three rods I2,

andare maintained'in spacedrelation by hollow spacers I5, I6, and nuts'I'I, also carried'on rods I2 vPlatte Il is a brass disc 1.844" in diameter and 0.070" thick, having a 0.250" diameter hole in the center toreceive insulator 6, three .070" holes around the periphery to receive the rods I2, and

an annular slot or counterbore 0.235 wide and 0.055"l deep, the center line of the slot being 0.592 radially from the center of the plate. The slot is illed with paraffin I9, until the surface o the paraffin is flush with that of the plate. TheV entire plate is covered with an aluminum foil 2'I, weighing 50 milligrams/ cm?.

" Plate 9 is of the same diameter as that plate, carries 'three similar 0.070 holes around its periphery 120 apart, and has a center hole 0.250 in diameter. But itis only 0.050 in thickness, and carries a much wider and shallower annular counterbore than that in plate 8. The counterbore is 0.0055 deep and 0.696 wide, and its center-line is 0.588 from the center of the plate. It vis'aiso oued with paraffin la until flush with the surface of the plate. A thin conducting coating 'such as carbon in the form of Aquadag, having electrical resistance of, preferab1y,` one megohm or less, is sprayed over the entire surface of parain I8.

The plates 8 and 9 are disposed 3.5 cm. apart within the counter shell. By judicious choice vof dimensions and pressure of the filling gas, protons produce in the counter pulses many times larger than the largest gamma-inducedy pulses, so that the gamma pulses may be discarded by a simple electronic discriminator, as shown hereinafter.

APlates 2, 3 are soldered to shell l, and nuts .I4 are soldered inplace vto plate 3 tomake the coun,- ter vacuumtight. evacuated for several days to a pressure of 10'.5 mm. Hg or lower, and lled thereafter with 13.2 cm. I-Ig of purified methane and 30.0 cm. Hg i' argon through filling tube 20. The tube should then be crimped, the open end soldered over, and the counter removed from the vacuum system.

Hydrogen or an hydrocarbon is preferred for the counting gas. Argon is added to the counter to increase proton pulse size, yet the gammaproduced pulses are kept below threshold level. Parain and the (CI-MN plastics are preferred hydrogenous proton radiators. The area of the hydrogenous slabs may be chosen to t the particular application; for example, for a small survey meter detector like that shown in Fig. 2, the area of the thin annular slab I9 maybe arbitrarily decided upon. Then the contribution to the counting rate of the slab alone may be determined for the entire spectrum of energies, assuming values of 2.6 103vfor K1, 1.72 vfor. n, 3 m. e. v.

for E1, and .02.m. e, v.- for B-inEquationsB and.

The counter should then bev 10-l 'libe cl'ioice.Y of valuesj forELandfB also den, be seen from` Equation 9; i. e.; the. slab thickness, in which a3 In. e. v. protonwill be slowed 15.0.0.2

term-ine. the thickness. offv the slab, as.. may

m. e. y".

' The sizeof thedetector chamber may also be.

chosensomewhat arbitrarily. In choosing cham--- ber length, the. energy lostb'y a ganima-produced electron having a range equalto the diagonal of` thev chamber, should be kept lower than theenergy. give up by the lowest energy protons .which areto be measured. In choosing the gas pressure, the contribution to the overallicoun'ting rate, which is desiredr fromV the lilling gasgdetermines, selectionof the desired number-of hydrogen atoms C, Equation` 7, which, together.. with the chamber. volume, determines the Y'retniired pressure.

As may be seen from curve C, Figure 1, thethick paraflin slab adds to the` counter-.- response aboveV 5 m. e. V. Therefore, the slab thickness ischsen greater than the range ofa' l0 m. e. vfproton,

determined from Equation 3. The tlfiicknessvof-l themetal foil desired is that required-to stop-all protons having energies 5 m. e. v. orless, and may be determined from Equation 13. The area of the thick radiator is determined by the magnitudeV or the contribution to counting rate desired, and the optimum value for this area has been determined to be 34%` the areaarbitrarily` chosen for the thin radiator.

One advantage oi my detector is that it may be readily combinedfwith simple commercially available electronic components to form a counting system. Figure 3 shows inblock form a system including a preferred form of my novel detector with which fast neutron dosage`to tissue can be measured, even in the presence of' gamma radiation. The counter SIIl is energized by a source of counter voltage 3l, whichv may furnish 1300 volts D. C. Pulses occurring in theV counter are amplified in pre-amplifier 32, which may be the A-l-B device described by Bell and Jordan in Rev. Sci. Instruments 18, l0 (194,7), and-are further amplied in linear amplifier 33, a preerred form of which is described in the same article. Pulse height discriminator 3d,v which` may also be of the type describedby Bell and Jordan, supra, allows only pulses above'a selected bias voltage to enter count-rate meterr35, where the number of pulsesper unit ytime may be'counted, indicated, and/or recorded.` The rate meter, which may be of any conventional type, Operates an` indicator calibrated'v in rep,. to indicate dosage directly. A bias of 19 volts on discriminator 35 has given very close agreement between experimental response, andV calculated dosage curves, when the counter is exposed to monoenergeticneutron beams of Various, energies.

Such elaborate electronic components arenoi necessary or desirable in aportable survey meter. Rather a simple three-stage D. C. amplifier of conventional design, accompanied by a singlestageY discriminator and a simple count-rate circuit 'are preferred. The power supply for detector and electron tubes may be furnished by batteries and/'or electronic supplies, as is convene. tionally done.

Nor is the design of my detector limited to the dimensions given in connection with Figure 2, or, to, the double-chambered tube there'` shown. Single chambered detectors are also lgood dosage measuring devices, though not so sensitive as the twin-chambered ones.. In av single chamber de.- Sign M11-in Slab 0f yinr-fiffmmar.f9r-ui--- 'wall o'fa cylindrical chamber 5 om.'V in diameterl and 3.5 cm. long. The outside plane surface of the 'paraffin is covered with aluminum foil, and a cylindrical slab of thick parain is placed n the other side of the foil. The inner slab might, for example, Weigh 13 mg./cm.2, the thick slab might be 100 ing/cm?, the foil may be 29 mg./cm.2, the methane gas be admitted to 30 cm. pressure, and the ratio of diameter of the thin slab to that of the thick slab may be 5.4, in accordance'with the above teachings. Y lIt will be apparent to those skilled in the ar that I have provided a novel counter particularly adapted t0 measure and indicate the dose to tissue from fast neutrons, and that I have further provided a counting system incorporating my novel counter tube with which I can measure neutroninduced dosage independently of gamma ray induced counting inaccuracies. By providing a proportional counter having two hydrogenous radiators: a thick ring at least as thick as the maximum proton recoil range in paraffin and a thin disc of specified dimensions, the areas of the two radiators being critically related; a predetermined number of hydrogen atoms per unit volume of filling gas; a metallic foil; and means for pulse-height selection, I can determine by direct measurement of fast neutron iiux substantially the radiation dosage to tissue which would result from that .'dux.

Having described a preferred embodiment of my invention in detail, I claim:

Y 1. A fast neutron dosimeter comprising a cylindrical shell, an electrode disposed axially thereof, a pair of end plates closing said shell, first and second shelf members provided with respective annular counterbores disposed within said shell in spaced relationship, first and second annular plane hydrogenous slabs disposed in the respective counterbores, a metallic sheet substantially covering the first of said slabs, an hydrogen-bearing gas disposed within said shell, the thickness and composition of said metal sheet being such that substantially all protons recoiling from neutrons of less than substantially million electron volts will lbe absorbed by said sheet, the thickness and composition of said second slab, the areas of said slabs, and the pressure of said gas being so related that the response of said detector varies with the energy of said neutrons as does the quantity EEiaiFiQ where E is the energy of one of said neutrons; ai is the scattering cross-section of the ith kind of atom in tissue;

Fi is the average fractional loss of energy per neutron collision with the ith kind of atom; Q is the number per unit volume of the ith kind of atom, and the summation is taken for hydrogen, oxygen, carbon, and nitrogen atoms in proportion as they occur in tissue.

2. A fast neutron dosimeter comprising an envelope deiining all save one wall of a counter chamber, an electrode disposed axially Within Said envelope, an hydrogen-bearing counter-nlli'ng gas, a rst plane hydrogenous slab forming said one wall, one surface of a metallic sheet disposed adjacent the outside of said one Wall, and a second plane hydrogenous slab of substantially greater thickness and smaller surface area than said iirst slab disposed adjacent the reverse surface of said sheet.

3. A fast neutron dosimeter comprising a gastight cylindrical envelope, a wire disposed axially thereof, rst and second shelves disposed in spaced relation within said cylinder and substantially perpendicular to the axis thereof, a flrst hydrogenous slab having one plane surface coated with a thin conductive coating disposed obverse surface to said first shelf, a second hydrogenous slab of greater thickness, smaller surfacearea, and substantially equal hydrogen content relative to said rst slab carried by said second shelf, a metal absorber foil carried by said second slab, an hydrogen-bearing gas disposed within said envelope, and conductive means for connecting a source of potential between said wire as one electrode and said coating and said foil as respective opposite polarity electrodes of the two counters formed within said envelope.

4. A neutron dosimeter which responds to fast neutrons substantially as does the dosage rate to human tissue comprising a proportional counter including an outer shell electrode and an axial electrode; a first hydrogenous slab of thickness substantially equal to the range of a 3 rn. e. v. neutron therein forming one wall of said counter; a metal foil disposed on the outside surface of said slab; a second hydrogenous slab thicker than the range of a 10 m. e. v. neutron therein disposed parallel to rst slab on the opposite side of said metal foil; an hydrogenous counter-nlling gas; the surface areas of said slabs and the pressure of said gas being so related that the number of recoil protons ejected at greater than a selected minimum energy within said gas by said neutrons varies with the energy E of incident neutrons as does the quantity EEwiFiQ, where ai is the scattering cross section of the ith kind of atom in human tissue, F1 is the average fractional loss of energy per neutron collision with the ith atom, Q is the number per unit volume Vof the ith kind of atom, and the summation 2i is taken for hydrogen, carbon, oxygen, and nitrogen atoms in relative proportion as they occur in human tissue.

5. A neutron dosimeter which responds to fast neutrons substantially as does the dosage rate to human tissue comprising a proportional counter including an outer shell electrode'and an axial electrode; a first hydrogenous slab of thickness substantially equal to the range of a 3 m. e. v. neutron therein forming one wall of said counter; a metal foil disposed on the outside surface ofr said slab; a second hydrogenous slab thicker than the range of a, 10 m. e. v. neutron therein disposed parallel to iirst slab on the opposite side of said metal foil; an hydrogenous counter-filling gas; the surface areas of said slabs and the pressure of said gas being so related that the number of recoil protons ejected at greater than a selected minimum energy within said gas by said neutrons varies vwith the energy E of incident neutrons as does the quantity EEicriFiQ, where ai is the scattering cross section of the ith kind of atom in human tissue, Fi is the average fractional loss of energy per neutron collision with the ith atom, Q is the number per unit volume of the ith kind of atom, and the summation 2i is taken for hydrogen, carbon, oxygen, and nitrogen atoms in relative proportion as they occur in human tissue, a power source for energizing said counter electrodes; a pulse amplifier connected to said counter electrodes; a pulse height selector adapted to pass only those pulses from said amplifier of magnitude greater than that corresponding to the selected bias energy; and means for counting the rate of occurrence of said pulses passed by said selector. said rate being a measure of dosage rate to human tissue.

6. A neutron dosimeter comprising a cylindrical shell; an axial electrode; first and second shelf members provided with respective annular counterbores and disposed in spaced relation within said shell; first and second annular hydrogenous slabs disposed in said respective counterbores; a metallic foil covering said rst slab; an hydrogenous gas disposed within said shell; and a thin, proton-pervious conductive coating disposed on the exposed surface of said second slab; the thickness of said rst slab being greater than the range of a 10 m. e. v. neutron therein/*the thickness of said second slab being substantially the range of a 3 m. e. v. neutron therein, *the thickness of said foil being of substantially vthe range of the 5 m. e. v. proton therein, the surface area of said first slab being substantially .34 times that of said second slab, the counter volume and the pressure of said gas therein being so related as to provide a predetermined number of hydrogen molecules in said counter.

GEORGE S. HURST.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS OTHER REFERENCES The Design of Neutron Counters Using Multiple Detecting Layers, Lowde, Rev. of Sci. Inst., vol. 21, No. l0, October 1950, pages 835-841. 

2. A FAST NEUTRON DOSIMETER COMPRISING AN ENVELOPE DEFINING A SAVE ONE WALL OF A COUNTER CHAMBER, AN ELECTRODE DISPOSED AXIALLY WITHIN SAID ENVELOPE, AN HYDROGEN-BEARING COUNTER-FILLING GAS, A FIRST PLANE HYDROGENEOUS SLAB FORMING SAID ONE WALL, ONE SURFACE OF A METALLIC SHEET DISPOSED ADJACENT THE OUTSIDE OF SAID ONE WALL, AND A SECOND PLANE HYDROGENEOUS SLAB OF SUBSTANTIALLY GREATER THICKNESS AND SMALLER SURFACE AREA THAN SAID FIRST SLAB DISPOSED ADJACENT THE REVERSE SURFACE OF SAID SHEET. 